The Gibson Brothers are one of the premier acts in bluegrass, but at the moment they’re not in bluegrass at all. They’ve just released an album, “Mockingbird,” that ranges far outside of the genre that has been their home, and they’ve gone electric and added a pedal steel player to their band to perform it on tour. They’ll highlight the new record at the Bull Run Restaurant in Shirley Sunday.

The seeds of “Mockingbird” were planted when the brothers, Leigh and Eric, sat down about a year ago to figure out their next project. They had released 13 bluegrass records over the course of their career and now they felt a little restless. So they started to think about recording some of the songs they’d written over the years that didn’t fit their bluegrass band, just to give them life.

As they continued mulling, they connected with Nashville producer David Ferguson, with whom they’d worked 20 years ago. “So we called Ferg,” says Leigh, who spoke along with Eric about the new record in a recent phone conversation. “It was very loose. We hadn’t even gotten a real song list together.”

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A short time later, Ferguson called back. He had talked to Dan Auerbach of Black Keys fame, who has become a prolific Nashville-based producer in his own right, and Auerbach had expressed interest in being involved in the project. In fact, he had suggested that he and the brothers get together to write songs, and then record what they came up with.

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The Gibsons took Auerbach up on his offer. They put the songs they had been talking about recording back on the shelf, went to Nashville, and co-wrote all but one of the 11 songs on their new album. “We built these all from the ground up around Dan’s recording studio kitchen table, with Dan and some other writers. Every day we wrote, there would be the two of us, Dan, and another writer,” Eric explains.

Co-producers Auerbach and Ferguson assembled a group of crack session musicians to help record the songs, and what resulted was a complete departure from the Gibson Brothers’ bluegrass wheelhouse. There’s stone country in lead-off track “Travelin’ Day,” there’s the Everly Brothers echo of “Special One”; “I’m a Better Man” goes full-blown soul, while “Not Gonna Be Tonight” has a soaring, Orbisonesque sound. “A couple of the songs almost make me think of Don Williams, whereas ‘Lay Your Body Down’ sounds more like an Eagles or Bob Seger song. I can hear influences all over that record,” Eric offers. “Mike Barber, our longtime bass player, said ‘This is closer to what you guys have been listening to in the van the last few years.’”

The Gibsons are celebrated for maintaining the legacy of brother harmonies in bluegrass, but the new songs called for a different singing style on their part. As Leigh explains it, “I think these songs sort of allowed me — and I heard it in Eric’s voice, too — to use a part of my voice that you don’t always get to use in bluegrass. That’s not saying that there’s not all kinds of emotion in bluegrass music, but typically bluegrass is sung in a higher key, so there’s certain resonances that you don’t get out of your vocals. This is a different side of what we’re able to do.”

The title of the record subtly points to that different side. It came out of a line in the song “Love the Land”: “Mockingbird, if you haven’t heard, there’s never been a song so sweet.”

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“That line just jumped out at me,” Eric says. “I just liked the sound of ‘mockingbird.’” He did a little investigating and discovered that a mockingbird can sing a variety of songs. “And I thought, wow, this album has a variety of songs on it, and we don’t just sing bluegrass. This maybe proves that we can sing other things, too.”

Are the Gibsons worried that “Mockingbird” will alienate part of their core audience? Leigh says that certainly wasn’t their intention. “I think people who are fans of us and keyed in to the writing and the themes are going to like this record. But if the only music somebody cares for at all is bluegrass, then this is probably not going to be the project for them. And it might offend them.”

Eric adds the brothers anticipated some negative reaction, although he thinks the positive response has far outweighed it. “But if you’re any kind of artist, if all you care about is not offending people, is that art? If you’re really going after something that excites you creatively, you have to do it. This opportunity fell out of the sky, and it was exciting for us.” So exciting, in fact, that somewhere down the road there may well be more of this sort of detour. “I’m enjoying it so much that I would be disappointed if we didn’t do something like this again at some point,” says Leigh. “It’s been a lot of fun.”